Rosin is a commercially valuable, naturally occurring material used in various products, such as, for example, adhesives, sealants, printing inks and sizing agents for paper. Pine trees yield rosin, and rosin is designated as either wood rosin, gum rosin, or tall oil rosin. The different designations are attributed to the source of the rosin. Wood rosin is obtained through the extraction of rosin from pine stump wood. Gum rosin is obtained through the tapping of living pine trees.
Tall oil rosin is a byproduct of the process for making wood pulp for paper. In this process, wood is digested with sodium sulfite and caustic soda. The sodium salts obtained produce, on acidification, a dark, almost black, oily liquid with a strong, unpleasant odor. This is crude tall oil. The composition of the tall oil varies with the method employed, but usually includes about 10% unsaponifiable matter, with a remainder of approximately equal proportions of fatty acids and rosin acids. The fatty acids in the tall oil rosin comprise oleic and linoleic acid.
The principal components of rosin are rosin acids. Rosin acids comprise abietic acid forms and their isomers. Abietic acid is the most prevalent rosin acid contained in rosin.
Rosin-based material compositions containing free rosin acid often form crystals. Rosin acids are readily crystallized, and the resultant crystals are insoluble in water and alkali and are associated with many problems such as plugging pipelines, filters, and strainers of process equipment. Although these crystals grow relatively slowly, if their growth is unimpeded, the crystals can increase viscosity, decrease stability, and often render the rosin-based material composition unsuitable for use.
Crystallization of rosin acids depends on the type of rosin-based material used. When the rosin acid content of the rosin-based material is fairly high, there is an increased tendency for the rosin acids in the rosin to crystallize. Inhibiting crystal formation extends the product shelf life and avoids the aforementioned handling difficulties. The tendency of rosin types to crystallize is, in increasing order: wood, gum, and tall oil rosin. Wood rosin typically has a rosin acid content of about 87%. Gum rosin typically has a rosin acid content of about 92% and has a greater propensity to form crystals than wood rosin. Tall oil rosin typically contains up to 90% rosin acids and is also more likely to crystallize than is wood rosin.
In the past, methods have been used to inhibit the formation of crystals in rosin-based material. These methods have included, disproportionation treatment, which involves contacting rosin-based material with an active disproportion catalyst at an elevated temperature for a period of time until the amount of rosin acids has been reduced. Catalysts traditionally used in the disproportion of rosin-based material include palladium, platinum, nickel, iodine, sulfur and the like. While effective in reducing the tendency of rosin-based material compositions to crystallize, disproportion treatment is costly due in part to the expense associated with the disproportion catalysts.
Another method used to inhibit the formation of crystals in rosin-based material involves contacting rosin-based material with formaldehyde in order to modify the rosin structure by disrupting its close packing, which is required for crystal formation. However, formaldehyde has been identified as a toxic substance, and its continued use is discouraged.
A need exists for a simple, safe, and cost-effective way to inhibit the formation of crystals of rosin acids in rosin-based material compositions without the use of costly disproportion processes or through the use of hazardous substances such as formaldehyde.